CAIRO (AP) — Egypt’s military-backed authorities on Thursday stepped up their crackdown on the liberal icons of the 2011 uprising against Hosni Mubarak, with security forces storming the headquarters of a rights group and arresting six activists, including a prominent youth organizer.

Hours after the early morning raid in downtown Cairo, a criminal court in the Egyptian capital acquitted Hosni Mubarak’s two sons and his last prime minister of corruption charges in a case arising from a land sale dating back to some 20 years ago. The sons remain in detention and still face other corruption charges.

Five of the six activists were released after nearly seven hours in detention. One of the five, lawyer Mahmoud Bilal, described in a news conference how plainclothes policemen blindfolded, beat and verbally abused them after binding their hands.

Already in detention are three of the 2011 uprising’s best known youth leaders — Alaa Abdel-Fattah, Ahmed Douma and Ahmed Maher — who are accused of breaking a recently introduced law that places draconian conditions on street protests and of assaulting police.

The raid and the acquittal of Mubarak’s sons and Ahmed Shafiq will likely lend credibility to growing suspicions that the government installed by the powerful military after its July ouster of Islamist President Mohammed Morsi is prepared to accommodate Mubarak-era figures to broaden its support base as it grapples with near-daily street protests from Morsi supporters and growing dissent by liberal and secular youth groups.

The need for support is all the more pressing because of a worsening economy, growing criticism of the government’s human rights record and next month’s nationwide referendum on a draft constitution penned by a 50-member panel appointed by Interim President Adly Mansour.

The Jan. 14-15 vote on the draft will be the first real test of the post-Morsi regime, which is hoping for a comfortable “yes” majority to enshrine its legitimacy. Morsi’s Muslim Brotherhood says it will boycott the vote and is calling on its followers to take to the streets during the two-day referendum.

Parliamentary and presidential elections — the last two steps in the road map announced by military chief Gen. Abdel-Fattah el-Sissi in July — are expected by the summer of 2014, although it has not been finally decided which will be held first.

Mansour, the interim president, on Thursday held the first of four meetings with politicians, associations, unions and youth groups at the end of which he is expected to announce a final timetable for the two votes.

The corruption acquittals also will likely be viewed as a continuation of the perceived failure to bring to justice those who committed crimes during Mubarak’s 29-year rule. To date, nearly all policemen charged in the killing of more than 800 protesters during the 2011 uprising have been acquitted or received suspended sentences. Several senior Mubarak regime figures have also been acquitted of criminal charges.

The Cairo criminal court found Gamal and Alaa Mubarak and Ahmed Shafiq innocent of corruption in a case that arose from the 1995 sale of a plot of land to Mubarak’s sons by an association led at the time by the former prime minister.

Prosecutors claim the land was sold to the two at below market value and were given a larger plot than what was stated in the contract. Also acquitted were four retired generals who served as board members of the association.

Alaa, a wealthy businessman, and Gamal, his father’s one-time heir apparent, are facing a separate trial on other corruption charges. They have been held since April 2011 — two months after their father stepped down in the face of the popular uprising.

A small group of Mubarak supporters in the courtroom cheered the verdict. “Oh Gamal Mubarak, the presidency awaits you,” they chanted.

Shafiq, a career air force officer like Mubarak, has lived in exile abroad since he was narrowly defeated by Morsi in a June 2012 presidential runoff. Security officials at Cairo’s airport say Thursday’s verdict repeals standing instructions that Shafiq must be arrested on arrival at any of the country’s entry ports, clearing the way for his return.

Shafiq’s lawyer, Yehya Kadri, told the private Al-Hayat TV channel that the former prime minister intended to return home but did not say whether he would seek office.

A statement by the court said Mubarak’s sons, Shafiq and other defendants had committed only a minor “financial irregularity,” and “administrative violation” that are not criminal. It also said that Alaa and Gamal Mubarak have given back the plot. It did not say whether the two could face any other penalties.

Morsi was toppled in a popularly backed military coup on July 3 and is on trial for inciting murder while awaiting a separate trial on charges of conspiring with foreign militant groups. Mubarak was sentenced to life imprisonment in 2012 for failing to stop the killing of protesters during the 2011 uprising that ended his rule. He was acquitted on appeal and is now being retried.

The airport officials spoke on condition of anonymity because they were not authorized to speak to journalists.

The activist still being held by police following the raid on the offices of the Egyptian Center for Economic and Social Rights is Mohammed Adel of the April 6 group, one of several youth movements that led the 2011 uprising.

Lawyer Gamal Eid said Adel and the five others from the center were seized along with computers during the raid. The five were released after being taken to a police station.

Eid said security officers pushed a gun inside the mouth of one of the five to silence him during detention, while beating another.

Bilal, the lawyer who recounted the raid and arrests in the news conference, said the six had been initially taken to an unknown location where they were lined up facing a wall. Every time one of them asked a question, asked to be taken to the toilet or protested the arrest, he was hit and insulted.

The tactics as described by Eid and Bilal evoke the Mubarak-era police brutality consistently cited as a main reason for the 2011 popular uprising.

A statement by 14 Egyptians rights groups denounced the raid, saying it pointed to a “new chapter of repression, dictatorship and autocracy in Egypt.”